Using the Mayan calendar, a uinal is 20 days, and a tun is 18 uinals. How many days are in 2 tuns?
One tun is $18$ uinals, or the same as $18\text{ uinals}\times\frac{20 \text{ days}}{1 \text{ uinal}}=360\text{ days}$. Thus, two tuns would be $2\times360\text{ days}=\boxed{720}\text{ days}$.